Answer:
1. Li said learning to speak French is easier than you think.
Step-by-step explanation:
An infinitive is when there is the structure or form "to + verb" in the sentence. In simple words, the infinitive is when the preposition "to" is followed by the simple/ base form of a verb.
In the given examples or sentences, an infinitive is present in sentence 1. The infinitive is "to speak", whereas there are no infinitives in the other sentences despite having the preposition "to".
Thus, the correct answer is option 1.